Bistable phototube



March 10, 1959 n D. R. YOUNG Y 2,877,355

BISTABLE PHOTOTUBE Filed Mrch 28, 1955 2 Sheets-Sheet 1 FIGJ. FIG.2

E 'IIIIIIIIIIIIIIIIA LIGHT SOURCE INVENTOR. DONALD R. YOUNG AGENT United States Patent BISTABLE PHOTOTUBE Donald R. Young, Poughkeepsie, N. Y., assignor to International Business Machines Corporation, New York, N. Y., a corporation of New York Application March 28, 1955, Serial No. 497,130 Claims. (Cl. 250-211) This invention relates to electron discharge devices and more particularly to an improved photosensitive discharge tube wherein light impulses are relied upon to initiate conduction and the conductive state is thereafter maintained by reciprocal action between a photosensitive cathode and an emissive anode.

Photosensitive tubes of this type have utility in controlling a train of events whenever the ambient light in a particular area reaches a predetermined intensity or when light impulses are applied for control purposes as a result of other conditions in a manner comparable to conventional grid control of a thyratron. Heretofore photosensitive discharge devices of this character have been found to operate in an unreliable manner and a principal obect of this invention is to provide a light responsive tube capable of positive response only upon application of a controlling light impulse having a level of light intensity above a predetermined magnitude.

Sporadic response of known devices is considered to arise primarily as a result of random energy transitions occurring Within the photosensitive and/or phosporescent materials with which the electrodes are coated; and an obective of the invention resides in providing a threshold of operable light intensity below which tube operation is prevented.

In accordance with the invention a bistable phototube is provided having a light quantum emitting anode and photoelectron emitting cathode with a clamp grid electrode positioned between these elements and functioning to establish the threshold and to prevent sporadic triggering action caused by random energy processes within or without the tube. Application of a light impulse of sufficient threshold intensity delivered by an external source causes the cathode to be activated to a degree such that the clamp grid is saturated and a number of high velocity electrons reach the anode with the light emitted therefrom maintaining the cathode sufficiently activated to continue tube conductivity until the applied cathodeanode voltage is reduced.

The clamp grid element is maintained at a positive potential with respect'to the cathode and collects substantially all electrons emitted therefrom due to external light stimulation below this threshold value as well as those random electrons developed from the activated cathode surface in the absence of external light.

A broad obect of the invention is to provide an improved phototube of increased stability in that sporadic action in the absence of triggering light impulses is prevented.

A more specific obect is to provide a bistable phototube having a photoemissive cathode electrode and light quantum emissive anode with a clamp grid electrode interposed to prevent accidental triggering.

Other objects of the invention will be pointed out in e following description and claims and illustrated in accompanying drawings, which disclose, by way of nple, the principle of the invention and the best mode,

2,877,355 Patented Mar. 10, 1959 ICC which has been contemplated of applying that principle.

In the drawings: Figure 1 ilustratesan embodiment of the phototube according to the invention wherein an opaque cathode and spiral clamp grid electrode are employed.

Figure 2 shows a further embodiment of the phototube wherein triggering light impulses are directed through a semi-transparent photo cathode and the clamp grid is in the form of an annular ring.

Figure 3 is a graph showing the spectral energy emission characteristic of calcium tungstate and willemite phosphors :along with the spectral energy response characteristic of SbCs, photosensitive material.

Figure 4 is a graph showing the persistance characteristic of calcium tungstate and of willemite phosphors.

Figure 5 is a schematic showing of the phototube and circuitry employed for its operation.

Figure 1 shows particularly an arrangement of electrodes mounted in conventional manner within an en velope 10 of glass or similar material and closed at the bottom by a glass stem. The cathode electrode comprises a pair of semi-cylindrical elements 11 forming an enclosure within which an anode 12 and a spiral clamp grid 13 are positioned. Activating light energy is applied from an external source such as a conventional Kerr cell (not shown) and is directed axially or through the space provided by the two cathode sections so as to impinge upon the inwardly disposed cathode surfaces.

A structural modification is shown in Figure 2 wherein electrodes comparable to those in Figure 1 are given similar designations. The cathode 11 comprises a semitransparent element through which the triggering light pulses are directed to the anode 12; with the grid element fabricated as a ring 13 and interposed between the anode and cathode.

In both structures, the cathode I1 is coated with photoemissive material and the anode 12 coated with a phosphorescent material so that once the cathode is activated by a light pulse and emits electrons, these electrons are attracted toward the anode causing light emission which maintains the cathode activated and the tube in a conductive state.

For a given matched pair of phosphor coated anodes and photoemissive cathodes, the energy level required to maintain conduction is determined by the anode-cathode voltage or the applied electric field. The spectral response for SbCs used as the photoemissive coating material for the cathode, is shown in Figure 3 along with the energy transmission characteristic for calcium tungstate and willemite phosphors, which may be used for coating the anode. While these latter materials are fairly closely matched to the most eflicient response range of SbCs calcium tungstate has a much shorter persistance characteristic as compared with that for willemite, as illustrated in Figure 4. In the actual tube construction a calcium tungstate phosphor was found more difiicult to process so that willemite has been used with better success, however, it is not intended that the present invention be limited to a device using these particular materials as other photoemissive compounds and phosphors may be matched and employed as electrode coating materials.

Due to the fact that the phosphor must conduct the current flowing through the device, its internal resistance may be reduced by preparing an admixture of carbon with the phosphor with the result that a reduced disintegration rate is obtained. Tube life may be further improved by the use of a large diameter anode with a relatively thin coating of the phosphor mixture. In the structure illustrated in Figure l the anode cylinder dimensions are 0.25 inch outside diameter, the grid 0.75 inch in diameter wound at 25 turns per inch and the caththe source 22 through by controls the current ode 1.0 inch inside diameter; with all electrodes approximately 1.0 inch long. Comparable surface areas are provided for the electrodes in the modification shown in Figure 2.

The tube is manufactured in the well known manner except as will be hereinafter described. Before assembly of the electrode, the cathode is coated with antimony and the anode with calcium tungstate or Willemite. The electrodes are then mounted in a conventional envelope having the usual getter tabulations with a 22 mgrn. caesium chromate silicon pellet in a nickel cap mounted in one tubulation. After evacuation, the caesium is released and the tube baked at a temperature conductive to opti mum SbCs, formation or up to 500 C.

As mentioned heretofore, the grid element 13 is provided in order to establish a threshold in the electron transfer efliciency between the anode and cathode and is biased to a potential equal to, or exceeding, the positive voltage of the anode through a very high impedance. For random photo currents or very weak light impulse signals, the electron current in the tube then divides b..- tween the anode 12 and grid 13. Since the impedance of the grid bias source is made high, the photo current developed by an intense light signal saturates the current carrying capacity of the grid circuit and emitted electrons reach the anode in greater number so that the electron transfer efiiciency approaches the value it would have in the absence of the clamp grid.

Referring now to Figure 5, a circuit arrangement is shown providing the proper operating potentials to the tube elements which are schematically illustrated. The anode 12 is connected through 0.5 megohm resistor 20 to a potentiometer 21 which is energized from a voltage source 22 developing a potential of about 2.5 kv. The

clamp grid 13 is coupled to the high voltage terminal of the potentiometer through a 50 megohm resistor 23. The cathode 11 is connected to the negative terminal of ground by a parallel connected capacitor 24 and resistor 25 of 100 K ohms. Output signals indicative of tube conduction may be detected across the resistor 20 or 25, or a load device may be connected in series with the anode-cathode circuit as described. The value of the resistor 25 determines the saturation current of the device in the fired condition of operation and theredensity acting to regulate tube life. The capacitor 24 may provide a means for turning off the device by an overload light pulse or to allow tube conduction to persist only for a predetermined period with each controlling light pulse applied to the tube. A For overload cut off operation an increased current flow causes a resultant voltage drop across the resistor 25, and the capacitor maintains the developed charge, which is in opposition to the anode-cathode voltage, for

a time long compared to the fall time of the overload I light pulse. For limited conduction operation, the resistor 25 is made large so that after the capacitor 24 discharges through the tube circuit the current fiow pro duces a voltage drop thereacross such that source 22 is unable to maintain a minimum anode-cathode potential.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention.

providing a threshold It is the intention therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A photoelectric tube comprising an envelope including a light permeable region, a cathode mounted within said envelope to receive light through said region, said cathode being coated with SbCs an anode arranged opposite said cathode to receive electrons emitted therefrom, said anode having the surface facing said cathode coated with willemite phosphor material, said surface being disposed so as to direct light emitted therefrom to said cathode for maintaining said cathode activated and a grid electrode mounted intermediate said cathode and said anode, a potential source connecting said anode and cathode, a circuit means connecting said grid electrode to the anode side of said voltage source through a high impedance said grid electrode providing a threshold below which conduction may not occur.

2. A photoelectric tube comprising an envelope including a light permeable region, a cathode mounted within said envelope to receive light through said region, said cathode being coated with SbCs an anode arranged opposite said cathode to receive electrons emitted therefrom, said anode having the surface facing said cathode coated with calcium tungstate phosphor material, said surface being disposed so as to direct light emitted therefrom to said cathode for maintaining said cathode activated and a grid electrode mounted intermediate said cathode and said anode, a potential source connecting said anode and cathode, and circuit means connecting said grid electrode to the anode side of said voltage source through a high impedance said grid electrode providing a threshold below which conduction may not occur.

3. A photoelectric tube comprising an evacuated envclope including a light permeable region, a cathode mounted within said envelope and adapted-to receive light through said region, said cathode being coated with a photoelectric material, an anode arranged opposite said cathode to receive electrons emitted therefrom, said anode having the surface facing said cathode coated with a phosphor material containing an admixture of carbon.

4. A photoelectric tube as set forth in claim 3 wherein a grid electrode is mounted intermediate said cathode and said anode.

5. A photoelectric tube as set forth in claim 4 including a source of potential, circuit means coupling said anode to the positive terminal of said source and said cathode to the negative terminal of said source, and

further circuit means coupling said grid to the positive terminal of said source through a high impedance in below which anode to cathode conduction may not occur.

References Cited in the file of this patent UNITED STATES PATENTS 1,678,077 Powell July 24, 1928 2,058,941 Arnhym Oct. 27, 1936 2,092,814 Schaffernicht Sept. 14, 1937 2,196,691 Batchelor Apr. 9, 1940 2,258,436 Von Ardene Oct. 7, 1941 2,322,361 Iams June 22, 1943 2,523,132 Mason et al Sept. 19, 1950 2,555,423 Sheldon June 5, ll 2,754,428 Franks et al July l0, 1956 2,789,240 Cohen Apr. 16, 1957 

1. A PHOTOELECTRIC TUBE COMPRISING AN ENVELOPE INCLUDING A LIGHT PERMEABLE REGION, A CATHODE MOUNTED WITHIN SAID ENVELOPE TO RECEIVE LIGHT THROUGH SAID REGION, SAID CATHODE BEING COATED WITH SSBCS4, AN ANODE ARRANGED OPPOSITE SAID CATHODE TO RECEIVE ELECTRONS EMITTED THEREFROM, SAID ANODE HAVING THE SURFACE FACING SAID CATHODE COATED WITH WILLEMITE PHOSPHOR MATERIAL, SAID SURFACE BEING DISPOSED SO AS TO DIRECT LIGHT EMITTED THEREFROM TO SAID CATHODE FOR MAINTAINING SAID CATHODE ACTIVATED AND A GRID ELECTRODE MOUNTED INTERMEDIATE SAID CATHODE AND SAID ANODE, A POTENTIAL SOURCE CONNECTING SAID ANODE AND CATHODE, A CIRCUIT MEANS CONNECTED SAID GRID ELECTRODE TO THE ANODE SAID OF SAID VOLTAGE SOURCE THROUGH A HIGH IMPEDANCE SAID GRID ELECTRODE MPROVIDING A THRESHOLD BELOW WHICH CONDUCTION MAY NOT OCCUR. 